Asthma is a relatively common illness worldwide. In 2009, the Centers for Disease Control estimated the asthma prevalence rate in the U.S. to be 8.2% of the population or 24.6 million people. Worldwide the burden of asthma is large, with an estimated prevalence of over 300 million cases and this number is expected to grow by more than 100 million cases by 2025.
Asthma is one of the country's most costly illnesses. The annual U.S. expenditures for health and lost productivity due to asthma are estimated at over $20 billion. In 2006 and 2007 in the U.S. there were approximately 3500 asthma-related deaths per year and over 250,000 deaths worldwide.
Asthma severity is classified according to the frequency and severity of symptoms, or “attacks”, the results of pulmonary function tests and the level of medications required to gain control of symptoms (NIH, “Guidelines for the Diagnosis and Management of Asthma, 2007”). Approximately 30% of asthma patients have mild, intermittent symptoms of asthma with normal pulmonary function tests. Another 30% of asthma patients have mild, persistent symptoms (two or more episodes per week) with normal pulmonary function tests. Forty percent of asthma patients have moderate-to-moderate-severe, persistent daily or continuous symptoms of asthma with abnormal pulmonary function tests. It is estimated that over 80% of the asthma-related health care costs are due to 20% of individuals with moderate-to-moderate-severe persistent and/or milder, treatment-refractory asthma. Thus, treatment-refractory asthma remains a significant unmet medical need.
Modulating the immune system has been pursued as a desirable approach to treat asthma, as well as a variety of other diseases and disorders, including, but not limited to, autoimmune disease, infection, allergy, inflammatory conditions, spontaneous abortion, pregnancy, graft versus host disease and cancer. T cells have been a target of such modulation. T cells are lymphocytes that participate in multiple immune system functions. Subsets of T cells such as helper T cells, cytotoxic T cells and suppresser T cells, mediate different immunologic functions. Natural killer T (NKT) cells are a subset of T lymphocytes that share surface markers and functional characteristics with both conventional T cells and natural killer (NK) cells. Unlike T cells, they recognize glycolipid antigens rather than peptide antigens.
NKT cells can be divided into three subsets: Type 1 which express an invariant T cell receptor and are CD1d-restricted (iNKT), Type 2 (NKT) which express varied T cell receptors, but are CD1d-restricted, and Type 3 which express varied T cell receptors and are not CD1d-restricted (NKT-like). Type 1 iNKT cells express a uniquely rearranged, highly conserved, semi-invariant TCR-α chain (Vα24-Jα18 in humans and Vα14-Jα18 in mice), which preferentially pairs with specific TCR-β chains (Vβ11 in humans or Vβ8.2, Vβ7 and Vβ2 in mice). They are highly conserved throughout animal phylogeny. This is in contrast to most T cell subpopulations, which have diverse sequences for their T Cell Receptors (TCRs). The invariant TCR of iNKT cells reacts with glycolipid antigens presented on the MHC-I-like protein CD1d on the surface of antigen presenting cells. A hallmark of iNKT cells is their capacity to rapidly produced a mixture of cytokines, including IL-4 and IFNγ, which are signature cytokines otherwise produced by T helper type I (Th1) and Th2 cells, respectively. Invariant NKT cells are sometimes referred to as “Classical NKT Cells”.
A unique feature of iNKT cells is that they recognize the marine sponge-derived glycolipid, α-Galactosyl-Ceramide (α-GalCer), presented on CD1d. This has been utilized to monitor mouse, non-human primate (NHP), and human iNKT cells by flow cytometry, by using α-GalCer-loaded CD1d tetramers. The mouse monoclonal antibody 6B11, which binds to the invariant loop of the human-iTCR, has also been used to monitor human and NHP iNKT cells.
iNKT cells represent a very small subset of the total T cell population in human and non-human peripheral blood. Their relative numbers can vary over one hundred-fold between normal individuals, representing anywhere from 0.01% to over 1% of CD3+ cells in humans, where the low end of the range represents the majority of humans.
Conventional T cells require exposure to foreign antigen in order to mature and acquire memory phenotype. Clonally-expanded populations of conventional memory T cells that are depleted through pathological events, or by pharmacological intervention, can only recover when new thymic emigrants, with identically rearranged TCRs, are exposed to similar or identical foreign or pathogenic antigen as the original insult. Such depletion of T cells can create a “hole” within the immune repertoire for a given individual. Unlike conventional T cells, iNKT cells have a continuous regenerative capacity. In humans, iNKT cell regeneration has been studied following bone marrow transplantation. Full recovery of iNKT cell numbers to baseline has been observed one month following peripheral blood stem cell transplant rescue for ablative therapy. iNKT cell depletion therapy, therefore, could be less risky than depleting other T cells.
iNKT cells develop in the thymus, similar to other T cells. Studies in mice show that iNKT cells, unlike conventional T cells, acquired a memory phenotype during their natural development by recognizing hitherto unknown, endogenous antigens presented on CD1d molecules, and without requiring prior exposure to foreign or pathogenic antigens. Due to their memory phenotype, they can be rapidly activated and expand within the peripheral immune compartment in response to exposure to foreign or endogenous glycolipid antigens presented by antigen-presenting cells (APCs).
iNKT cells share characteristics of both the innate and adaptive arms of the immune system and thus play a unique role by modulating T and B cell responses as well as innate immunity (1). iNKT cells are rapid-onset which is a feature of the innate immune system. They also display features of the adaptive immune system because they share properties of other T cells such as antigen specific responses. As such, they serve as a bridge between the two systems where they can play both pro-inflammatory and immuno-regulatory roles either to enhance or attenuate developing immune responses, respectively (2).
The properties of iNKT cells has prompted investigations into the manipulation of iNKT cell function as a treatment for disease (3, 4, 5, 6). Numerous studies have shown that iNKT cells can regulate the balance between Th1 and Th2 responses. These cells are postulated to play a role in the response to pathogens, in immune surveillance in cancer, and in the regulation of autoimmunity. For most of these conditions, the iNKT cell defect has only been partially characterized and in some cases has been disputed by contradictory studies. Human studies, in particular, are constrained by two important limitations. First, most human studies have used suboptimal methods for the identification of iNKT cells. Second, most human studies are qualitative only, and little human data exists respecting the functional consequences of modulation of iNKT cell numbers, ratios, or function.
iNKT cell function has received significant attention concerning its potential role in asthma. Inhibition of iNKT cell function, either through genetic ablation of iNKT cells, or through pharmacologic blockade, blocks the development of airway hyper-responsiveness (AHR) in mice (7, 8). This is seen with both ovalbumin- and ozone-induced AHR (9). iNKT cells increase in the lungs of wild type mice following exposure to ovalbumin or ozone. In mice that are genetically devoid of iNKT cell function, through knockout of the gene encoding a portion of the invariant T cell receptor, adoptive transfer of iNKT cells restores the ability for allergens to induce AHR (9). Conversely, activation of iNKT cells in the airways with the potent iNKT cell agonist, α-GalCer, induces a response indistinguishable from AHR in both mice and non-human primates (10, 11). Mice genetically devoid of iNKT cells are also resistant to development of pathologic changes in the airways in a chronic Sendai virus model of COPD (12). Similar to the findings from the murine models, in humans, iNKT cells are not present in the lungs of healthy individuals, but they are present in the lungs of patients with mild-to-moderate asthma, and they increase further after allergen or viral challenge (3, 13, 14). There appears to be little or no correlation between the levels of iNKT cells in the BAL fluid and the peripheral blood of patients with asthma. The prior art has not established whether iNKT cells are causally connected to asthma or are the result of asthma or whether a therapy aimed at blocking iNKT cell function could be a treatment for asthma.
The mouse monoclonal antibody 6B11, which recognizes the human invariant T cell receptor, is the subject of US published application No. 2007/0160600. This antibody selectively binds human iNKT cells, being able to distinguish iNKT cells from other lymphocytes and other human tissue types. The application offers conflicting statements as to whether the 6B11 antibody would enhance or suppress an immune response. The application indicates at one point that the antibody stimulates proliferation of iNKT cells, suggesting that the antibody is an agonist of iNKT cells. The application suggests at another point that an increase in iNKT activity can result in a suppression of an immune response. At still another point, the application indicates that interfering with iNKT activity will suppress an immune response. In particular, the application suggests that an antibody should be coupled to a toxin to deplete iNKT cells in order to suppress an immune response. This application, therefore, does not make clear the medical circumstances in which use of the 6B11 antibody would be desirable.